Membrane cells are the state-of-the-art equipment for electrochemical reactions such as the electrolysis of sodium chloride to make chlorine and sodium hydroxide. Some of the cells have very large membranes. Plate-and-frame cells can have an active membrane area of up to 1.5.times.3.7 meters, which means that the membrane area sealed by the gaskets is even larger. Sealing is a particularly difficult task for the larger membranes, and these cells frequently use ribbed gaskets, with a width of up to 5 cm, between the plate and the membrane.
It is necessary to use a sealant between the gaskets and the membranes at the time the cells are assembled under compressive force, for three reasons:
1. The sealant prevents leakage of the electrolyte during operation. This is particularly important when using state-of-the-art membranes which contain small channels parallel to the dimensions of the planar surface, as taught in U.S. Pat. No. 4,437,951. The sealant must block the end of these channels, and must block capillary leakage down the length of multifilament membrane reinforcement.
2. The sealant lubricates the gasket/membrane interface so the force of closing the cell does not tear the membranes. The use of reinforced gaskets reduces the deformation of the gaskets but does not eliminate the need for lubrication.
3. The sealant provides a release function so the membranes are not damaged during disassembly.
Among the sealants now in use is silicone rubber, which is applied in solution and crosslinks on contact with air. The sealant generally must be allowed to cure for about five hours or more (until it loses its tackiness) without contacting any other surface. This makes application of sealant to vertical gasket assemblies difficult, because there is only a limited amount of free space available when the clamps are fully separated. The silicone sealant is flammable, does not seal membranes with channels from sacrificial fibers, and does not provide adequate release properties. Spilled sealant cannot be removed with water.
Du Pont Krytox.RTM. fluorinated grease has advantages over silicone rubber. It is effective as a release agent, can seal membranes with small channels made by removal of sacrificial fibers in the reinforcement of the membrane when using ribbed gaskets, and does not require air drying. This means that in a multicell electrolyzer, the Krytox.RTM. can be applied to each gasket and the coated gaskets may touch each other overnight prior to insertion of membranes. This is particularly important with very large cells, where vertical application of gasket sealant is required because the cell frames are too large and heavy to permit horizontal assembly. Further, the Krytox.RTM. grease is not flammable.
However, Krytox.RTM. grease must be applied with care because spilled sealant cannot be removed with water. Also, achieving leak-free performance is difficult when membranes are reinforced with fabrics made from multifilament yarns and when membranes are installed with flat gaskets.
An improved sealant with the following attibutes is desirable:
1. a low enough viscosity to ease application, but high enough to avoid running, particularly on vertical surfaces, after application;
2. ability to flow into voids, penetrating and plugging channels remaining after removal of sacrificial fibers in the membrane reinforcement and sealing against capillary leakage along the length of multifilament reinforcement fibers, and/or ability to plasticize to a degree a cation exchange membrane so that, upon cell closure, the pressure will urge the cation exchange polymer itself against and even into any voids.
3. effectiveness on flat as well as ribbed gaskets;
4. ease of spill cleanup, preferably with water;
5. compatablity with other sealants, such as Krytox.RTM. fluorinated grease, so that the two sealants can touch each other without detriment if the two are being used in the same cell;
6. chemical compatibility with the reactants, materials of construction of the cell, the products and the electrolytic process in general; and
7. cure time low or not required so that adjacent gaskets with sealant applied can touch promptly after application, so an entire large cell can be fitted with gaskets and sealant in one day and assembled with wet membrane the next day.
The sealing composition of the present invention has the desired attributes and is fully suitable for application on gaskets in the vertical as well as horizontal mode. It is particularly effective when applied with the applicator of the present invention.